This invention relates to improved electrodes for a membrane electrode assembly (MEA) for use in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC), and more particularly a method of manufacturing platinum (Pt) and platinum-ruthenium (PtRu) based membrane electrode assemblies using a filtration process incorporating carbon nanomaterials, such as carbon nanotubes.
A fuel cell is a device that converts the chemical energy of a fuel and an oxidant directly into electricity without combustion. The principal components of a fuel cell include electrodes catalytically activated for the fuel (anode) and the oxidant (cathode), and an electrolyte to conduct ions between the two electrodes, thereby producing electricity. The fuel typically is hydrogen or methanol, and the oxidant typically is oxygen or air.
Fuel cells are electrochemical devices that convert chemical energy directly into electrical energy. Compared with internal combustion engines, fuel cells are not limited by the Carnot cycle and in principle could have higher efficiency. With pure hydrogen as the fuel, fuel cells are very environmentally friendly. The combination of high efficiency, low environmental impact, and high power density has been and will continue to be the driving force for vigorous research in this area for a wide variety of applications such as transportation, residential power generation, and portable electronic applications. For portable electronic applications, important features include high power density (i.e., longer battery life) and compactness.
Silicon-based microfabrication technology is amongst the promising approaches for fabrication of compact micro fuel cells. However, the current methods for making electrodes for fuel cells, which typically includes spraying and/or brushing of platinum supported on carbon powder, is incompatible with microfabrication techniques. Therefore, there is need for improved electrodes and methods of preparing such electrodes for PEMFCs and DMFCs.
Direct methanol fuel cells (DMFCs) have attracted enormous attention as a promising power source for portable electronics applications such as laptop computers and cell phones. The interest in commercializing DMFCs is in part due to the fuel cell's simple system design, high energy density and the relative ease with which methanol may be transported and stored, as compared with hydrogen. In the state-of-the-art DMFCs, platinum supported on a carbon substrate is configured in the cathode as a catalyst for activating the oxygen reduction reaction (ORR). A platinum-ruthenium alloy is usually used as the anode electrocatalyst, and may be supported on a carbon substrate. The electrolyte is usually a perfluorosulfonate membrane, for which NAFION (available from DuPont) is a commonly utilized commercially available membrane. One of the major problems encountered in DMFCs is methanol crossover from the anode to the cathode. The permeated methanol causes “poisoning” of the cathode platinum catalyst and depolarization losses due to the simultaneous oxygen reduction and methanol oxidation on the platinum catalyst.
Reference is made herein to the well-known rotating disk electrode, which is used in the testing of the present invention as described below. As will be appreciated by those of ordinary skill in the art, the rotating disk electrode (RDE) consists of a disk on the end of an insulated shaft that is rotated at a controlled angular velocity. Providing the flow is laminar over all of the disk, the mathematical description of the flow is surprisingly simple, with the solution velocity towards the disk being a function of the distance from the surface, but independent of the radial position. The rotating disk electrode is used for studying electrochemical kinetics under conditions, such as those of testing the present invention, when the electrochemical electron transfer process is a limiting step rather than the diffusion process.
Polymer electrolyte based low temperature fuel cells, with their two best known variants, proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC), have been considered promising for powering automobiles, homes, and portable electronics. Their successful commercialization is, however, very much dependent on the activity and durability of their electrocatalysts. At present, all pre-commercial low temperature fuel cells use supported Pt and Pt alloys as their electrocatalysts. The critical properties to consider when choosing an electrocatalyst support include its electrical conductivity, surface area, macro-morphology, microstructure, corrosion resistance, and cost. Carbon black (CB), such as Vulcan XC-72, has been the most widely used electrocatalyst support because of its reasonable balance among electronic conductivity, surface area, and cost. Recently, many nanostructured carbon materials with graphitic structure, such as nanotubes (CNTs), nanofibers (CNF) nanocoils, nanoarrays and nanoporous hollow spheres, have been studied. Among them, CNTs are of particular interest due to their unique electronic and micro and macro structural characteristics. CNTs have also been shown to be more corrosion-resistant than CB under simulated fuel cell operation conditions.
Among the two variants of low temperature fuel cells, DMFCs have been attracting great attention for powering small devices, such as laptop computers, cell phones, and personal digital assistants, because of their high energy density, ease of handling liquid fuel, and low operating temperature. However, the slow electrokinetics of the anode reaction—a methanol oxidation reaction—is still a key problem to the commercialization of DMFCs. Normally, expensive noble metal alloys, typically Pt—Ru, with a high electrode metal loading (e.g., >2.0 mg/cm2) are employed in order to offer a reasonable fuel cell performance (e.g., 80 mW/cm2 at cell temperature of 90° C. and O2 pressure of two atmospheres). It has long been desired for a high performance anode catalyst to be developed so that the electrode metal loading and thus the cost of DMFCs can be reduced.
Some early investigations have found that, by simply replacing CB with CNTs in the conventional ink-paste electrode fabrication method, superior DMFC performance can be obtained. For example, a DMFC single cell with cup-stacked CNTs supported Pt—Ru anode catalyst showed nearly three times the maximum power density of a DMFC with CB (Vulcan XC-72) supported Pt—Ru anode catalyst, and it was suggested that CNTs can provide better charge and mass transfer.
Several types of carbon nanotubes may be used as electrocatalyic supports for low temperature fuel cells, for example, single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs) and double-walled carbon nanotubes (DWNTs). SWNTs can have a large surface area (e.g., 500-1000 m2/g) due to their small diameter (e.g., one nm), which is a favorable property as catalysts support. However, they normally contain a significant amount (e.g., two-thirds) of semiconducting tubes, which are poor electron conductors and thus are expected to be a poor electrocatalytic support. MWNTs are highly conducting, but they have limited surface area (e.g., 100-200 m2/g) due to their large diameter (e.g., forty nm). It was recently shown that most DWNTs are conducting tubes and that they can have high surface areas (e.g., 500-1000 m2/g). Thus a natural and logical choice for an electrocatalyst support is DWNTs.
Accordingly, there is a need for, and what was heretofore unavailable, an improved membrane electrode assembly incorporating filtered and/or oriented carbon nanomaterials.